Air Handler Units and Coils Flashcards

What is the correct procedure if an evaporator coil is found to be completely iced over during a service call?

Answer: Turn the system to fan-only mode to thaw the coil before diagnosing the cause

The coil must be allowed to thaw completely (using fan-only mode) before diagnosis because testing a frozen system gives inaccurate readings and won't reveal the true cause.

What is the difference between a 'chilled water coil' and a 'direct expansion (DX) coil' in an air handler?

Answer: Chilled water coils circulate cold water through tubes; DX coils use refrigerant directly

A chilled water coil uses chilled water pumped from a chiller as its cooling medium, while a DX coil has refrigerant flowing directly through its tubes from the compressor system.

What is the purpose of 'subcooling' in a condenser coil?

Answer: To cool liquid refrigerant below its condensing temperature to prevent flash gas in the liquid line

Subcooling cools liquid refrigerant several degrees below its condensing temperature, preventing it from flashing into vapor in the liquid line before reaching the expansion device.

What is a 'multi-speed' blower motor and what advantage does it provide?

Answer: A motor with multiple windings that can run at different RPMs to match varying system demands

A multi-speed blower motor has multiple tapped windings that allow it to operate at different fixed speeds, enabling adjustment for heating vs. cooling or different duct system conditions.

When replacing an evaporator coil, why must the refrigerant circuit be properly evacuated before recharging?

Answer: To remove air and moisture that entered the system during the open coil replacement

Evacuation removes air and moisture that entered the open refrigerant system during coil replacement; moisture in the system can cause acid formation, freezing at the metering device, and compressor damage.

What is the function of a 'coil baffle' or 'coil pan liner' in an air handler?

Answer: To prevent air bypass around the sides of the coil

Coil baffles seal the gaps between the coil and the air handler cabinet to prevent unconditioned air from bypassing the coil, ensuring all air passes through for proper treatment.

What is the primary advantage of a microchannel heat exchanger coil compared to a traditional round-tube plate-fin (RTPF) coil?

Answer: Microchannel coils have smaller refrigerant volume, lower weight, and better heat transfer efficiency

Microchannel coils use flat multi-port aluminum tubes that require less refrigerant charge, weigh less, and provide superior heat transfer due to their large surface area-to-volume ratio.